Epub3 has just recently been finalized as an eBook format, and there’s already a tool you can use to make sure your file works right.
Are you planning to sell your eBook in iBooks or other eBookstores? Then you’ll need this tool. It checks whether your eBook meets the requirements for Epub and Epub3, and it can tell you how your eBook meets the specs s well as what you need to fix. The IDPF has volunteered the space on their servers, and they’re making the tool available to all.
But you might want to wait a few more days before using this tool. This validator is based on EpubCheck by Threepress Consulting, and a new version was just released this weekend. It is not yet online.
This online validator is also limited to files smaller than 10MB, so that might hamper your efforts; thanks to audio and video Epub3 files can easily hit 50MB or more. Luckily you can also download a copy and run it on your own computer.